Liquid ink printers of the type frequently referred to as continuous stream or as drop-on-demand, such as piezoelectric, acoustic, phase change wax-based or thermal, have at least one printhead having drop ejectors from which droplets of ink are directed towards a recording sheet. Within the printhead, the ink is contained in a plurality of channels. Power pulses cause the droplets of ink to be expelled as required from orifices or nozzles at the end of the channels.
In a thermal ink-jet printer, the power pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in the associated channel and initially the ink bulges from the channel orifice. The bubble quickly collapses and the ink within the channel then retracts and separates from the bulging ink thereby forming a droplet moving in a direction away from the channel orifice and towards the recording medium whereupon hitting the recording medium a dot or spot of ink is deposited. The channel is then refilled by capillary action, which, in turn, draws ink from a supply container of liquid ink. Operation of a thermal ink-jet printer is described in, for example, U.S. Pat. No. 4,849,774.
The ink jet printhead may be incorporated into either a carriage type printer, a partial width array type printer, or a page-width type printer. The carriage type printer typically has one or more relatively small printheads containing the ink channels and nozzles. The printheads can be sealingly attached to one or more disposable ink supply cartridges and the combined printheads and cartridge assembly is attached to a carriage which is reciprocated to print one swath of information (equal to the length of a column of nozzles), at a time, on a stationary recording medium, such as paper or a transparency. After the swath is printed, the paper can be stepped a distance equal to the height of the printed swath or a portion thereof, so that the next printed swath is contiguous or overlapping therewith. This procedure is repeated until the entire page is printed.
In a typical ink-jet printing machine, the carriage must transport the printhead assembly across the page for printing as the recording medium is held stationary. After the printhead has scanned across the medium, the medium is advanced by a transport which typically includes a transport roller driven by a gear assembly which is in turn driven by a motor. In one example of an ink jet printer, the motor is a stepper motor which provides for accurate control of the medium advance by being coupled to the gear assembly which includes a number of gears for reducing the advance of the motor by the appropriate amount to, for instance, print at 300 dots per inch (dpi) or 600 dpi.
An ink jet printer's paper advance mechanism must be accurate to reduce mismatch between swaths. Among other things, this means reducing forces competing for the paper during imaging. One major force is that required for feeding. Some printers disengage the feed rolls by using the advance motor to retract the feed platform after the paper is in the advance nip. This is effective, but can compromise advance accuracy or constrain feed capacity. Thus, there is a need to reduce media drive loads during printing.
Various methods and apparatus of printing with liquid ink printers having an advance mechanism including a transport are described in the following disclosure which may be relevant to certain aspects of the present invention.
In U.S. Pat. No. 4,491,854 to Habelt et al., a printer with a guide ruler for flattening a record carrier is described. The record carrier is fed around a transport roller and over an elongated flat supporting surface extending alongside the roller, with a printing head displaceable along that surface. The transport roller can be rotated either by means of a handwheel connected thereto or by means of a motor, via a gearwheel drive.